1. Field of the Invention
The present invention relates to an acrylic rubber composition which is suitable for use as a sealing member while in contact with metal in automobile-mounted engine oil filters, distributors and the like.
2. Description of the Related Art
The problem of sticking between metal members and rubber members, after long-term contact of the metal members with the acrylic rubber compositions, has been addressed by subjecting the molded rubber members to post-treatment to form anti-stick coatings on the rubber member surfaces. Such an approach is described in Japanese Unexamined Patent Publication No. 5-148476. Since the step of forming the anti-stick coatings must be performed separately, manufacturing costs for such rubber members has increased.
Japanese Unexamined Patent Publication No. 6-157826 (published Jun. 7, 1994) discloses an acrylic rubber composition to be used as a metal-contacting member. The composition is prepared by combining an acrylic rubber with chromium, manganese, iron, cobalt, nickel, copper, zinc, aluminum, tin, lead, tellurium or a metal compound thereof, or with at least one type of phosphorus compound. This phosphorus compound helps prevent the rubber from sticking to contacted metals and protects the metals from corrosion. The phosphorus compounds can include inorganic compounds such as phosphoric anhydride and tris(nonylphenyl) phosphite, etc.
According to this document, at high temperatures, the acrylic rubber composition generates acidic substances such as hydrochloric acid or carboxylic acid. These acidic substances dissolve the poorly reactive iron oxide (black rust) which has usually formed on the surface of the metal member. Once the iron oxide has been dissolved, the highly reactive iron on the surface of the metal member is exposed. When this occurs, the surface of the metal member reacts with the phosphorus compound in the acrylic rubber composition, thus forming a phosphorus inert (poorly reactive) coating on the surface of the metal member. This inert coating is believed to prevent sticking between the acrylic rubber composition and the metal member.
However, upon close examination of this reaction, the present inventors have found that the anti-sticking effect of the phosphorus inert coating tends to be reduced when the acrylic rubber composition and the metal member are in contact for long periods of time. Upon long-term use, the acidic substances generated from the acrylic rubber composition, particularly highly oxidizing hydrochloric acid, damage the phosphorus inert coating formed on the surface of the metal member. As this coating deteriorates, the anti-sticking effect between the acrylic rubber composition and the metal member is reduced.
In light of these circumstances, it is an object of the present invention to obtain an acrylic rubber composition which effectively prevents damage to phosphorus inert coatings formed on the surface of metal members.